While much has been published or patented on the subject of PMA based catalysts and their uses as oxidation catalysts and, reactivation of other catalysts is generally known, very little prior work directed toward the reactivation of PMA based catalysts has been reported. One such method that is known is disclosed in Japanese patent application, No. 77/29,660, filed by Mitsubishi Rayon Co., Ltd., which describes the regeneration of a PMA based catalyst by treatment with ammonium hydroxide, and hydrogen peroxide or ozone and optionally also with nitric acid or ammonium nitrate. The inventors reported that the initial oxidation of methacrolein with the fresh catalyst gave 88% selectivity to methacrylic acid with 66% conversion of methacrolein; the spent catalyst gave only 71.5% selectivity to methacrylic acid with 30.3% conversion of methacrolein and, that following regeneration with ammonium nitrate and ammonium hydroxide and 5 parts of 30% hydrogen peroxide for 30 minutes at 90.degree. C., the catalyst gave 88.1% selectivity to methacrylic acid with 65.1% conversion of methacrolein. This process is undesirable inasmuch as it merely attempts to oxidize the deactivated catalyst and to do so outside of the reactor, necessitating shut-down.
Several other Japanese patents exist which disclose the use of ammonia for regeneration of spent PMA catalysts. However, none of which we are aware is directed toward reactivation of PMA based catalysts with nitrogen oxides.
Use of nitric oxide to reactivate a supported palladium catalyst containing 1% palladium has been described in Japan Kokai 75, 75, 587 wherein full reactivation was obtained by heating the catalyst with acetic acid in the presence of a nitrogen containing oxidant followed by treatment with potassium acetate. The latter treatment is indicative of a reconstructive regeneration. German Offenlegungsschrift No. 2,126,007 sets forth a regeneration scheme for a spent aluminum oxide/boric oxide rearrangement catalyst by heating while fluidizing with air containing nitrogen oxide.
In more recent work, conducted in the laboratories of the Assignee of record herein, PMA based catalysts have been regenerated in the presence of ammonia and hydrochloric acid. Such a process is referred to as corrosive reconstruction wherein a reconstructive transformation occurs generally involving the breakdown of one crystal lattice and the reorganization of another. Unless catalyzed in some manner, such reconstructive transitions have an appreciable activation energy compared to displacive transitions wherein only relatively minor shifts of atoms occur. It is believed that displacive transition would permit a deactivated catalyst to be reactivated in situ with a single oxidizing gas.
Thus, the prior art of which we are aware has not set forth a method by which a deactivated PMA based catalyst can be reactivated with only a gaseous oxide of nitrogen or mixtures thereof. To be useful, we believe the reactivation of PMA based catalysts must occur in the vapor phase, and whether inside or outside of the reactor, without catalyst removal. It is also necessary to reactivate the catalyst in a manner that is compatible with a fluid-bed process. It would be further useful to be able to treat the catalyst during on-stream use thereby maintaining its activity over greater periods of time.